2. Field of the Invention
This invention relates to an electrically conductive zirconia-based sintered body and a process for the production thereof, i.e. a zirconia-based sintered body which is not only excellent in mechanical properties such as strength, toughness, hardness, etc., but also has electric conductivity even in a low temperature region, and a process for the production thereof.
2. Related Art
Improvement of zirconia-based sintered bodies for toughness and high-temperature strength has been conventionally studied in an attempt to use them in cutting tools and materials for internal combustion engines, and partial-stabilized zirconia with Y.sub.2 O.sub.3, MgO or CeO.sub.2 has been recently developed. Due to technical limitations in manufacturing zirconia-based sintered bodies, however, all of the zirconia-based sintered bodies have simple shapes. For this reason, it is necessarily required to add a machining process to obtain a final product. Since, however, zirconia-based sintered bodies are insulating materials, they have a disadvantage that an electrical discharge machining method, which is excellent in machining efficiency, cannot be applied thereto at low temperature.
As a method for imparting zirconia with electric conductivity and making possible electrical discharge machining, there is known a method of incorporating an electrically conductive powder of TiC, TiN, etc., into a zirconia powder and sintering the mixture.
For example, Japanese Patent Publication No. 59266/1986 discloses a sintered body comprising ZrO.sub.2 and TiN. Concerning allowable impurities, however, it describes the following: The amount of SiO.sub.2 is up to 3%, that of Fe.sub.2 O.sub.3 or TiO.sub.2 is up to 0.5%, the total amount of impurities is up to 3%, and if impurities are contained in an amount exceeding these limits, densification is hampered and the resultant sintered body has poor toughness.
Further, Japanese Patent Kokai (Laid-Open) No. 103078/1985 discloses a sintered body with Y.sub.2 O.sub.3 or MgO as a stabilizer containing a carbide. Japanese Patent Kokai (Laid-Open) No. 138359/1987 teaches that one of carbide, nitride and carbonitride of Group IVa, Va or VIa element of Periodic Table is incorporated into ZrO.sub.2 as an electric conductivity-imparting agent and that 0.1 to 10% by weight, based on the agent, of oxide of an element identical with that of the agent is added for the purpose of improvement of sinterability, thermal conductivity and electrical conductivity.
However, these sintered bodies have difficulty in achieving electric conductivity without degrading toughness, hardness, mechanical strength, etc., which zirconia inherently has. It has been therefore desired to develop sintered bodies having good electric conductivity thereby to make electrical discharge machining possible at ordinary temperature and having excellent toughness, hardness, mechanical strength, etc.
Meanwhile, it has been recently reported to use TiO.sub.2 as a stabilizer of zirconia.
For example, K. C. Radford, et al report that TiO.sub.2 acts as a sintering aid for Y.sub.2 O.sub.3 -based zirconia (J. Mat. Sci., Vol. 14, p. 59 et seq., 1979).
Tsukuma studies the action of TiO.sub.2 as a stabilizer for TiO.sub.2 -Y.sub.2 O.sub.3 -based transparent zirconia (Kino Zairyo; 10, 1986, pp. 31-37).
It is, however, reported that since TiO.sub.2 stabilizes tetragonal system, it becomes difficult to cause stress-induced transformation with an increase in an amount of TiO.sub.2, and strength and toughness are decreased.
On the other hand, Japanese Patent Kokai (Laid-Open) Nos. 122161/1986 and 144167/1988 describe that incorporation of TiO is effective to achieve high strength of zirconia-based sintered bodies.
However, these patent documents neither are concerned with zirconia-based sintered bodies having electric conductivity nor mention toughness relative to incorporation of TiO.sub.2. Nor do they disclose how TiO.sub.2 behaves in the presence of an electric conductivity-imparting agent.
The present inventors have set a purpose, as conditions required to process and use zirconia-based sintered bodies, in providing a zirconia-based sintered body having a mechanical strength of at least 50 kg/mm.sub.2, having a fracture toughness value of not less than 4 MPam.sup.1/2, a Vickers hardness of not less than 1,000 kg/mm.sup.2 and a volume resistivity of not more than 1 .OMEGA.cm, and having sinterability at normal pressure and low temperature, and process for the production thereof.